Industrial waste water effluents present problems of purification which are quite different from the problems of municipal sewage treatment. The waste streams from each industrial process type is unique to that process and requires specialized rectification. Generally, neutralization of acidic or alkaline waste materials to obtain a pH between approximately 6-9, while of extreme importance in water pollution control, may be readily achieved by direct chemical treatment. The removal, solubilization or passivation of specific contaminants presents problems more difficult of solution, which ofttimes require a combination of physical, bio-chemical and chemical treatments. Especially trying is the problem of preventing color contamination of surface water into which industrial waste streams are discharged. Color contamination of surface water presents a visible asthetic problem as well as the problem of altered light penetration of the surface water.
In the pulping industry, the brown color of pulping effluents is largely the result of extracted tannins, lignins and their derivatives removed from the cellulosic pulp by washing of the pulp after selected chemical treatment stages. The bulk of the colored material is removed from the cellulosic bleaching process in the caustic extraction stage, which removes from the cellulosic material the previously solubilized non-cellulosic content of the pulp. Lignin and its chlorinated and oxidized derivatives are highly resistant to microbiological degradation and a color inherent in such lignin extracts pass through most biological treatment stations into the surface water discharge. Although chemical treatment of the colored bodies in pulp mill effluents is a very effective technique for reducing the content of colored bodies, the economics of such systems are exorbitant and therefore impractical in modern day technology. Therefore, chemical removal of colored material is not generally applied to pulp mill effluents in practice. The use of activated sludge facilities may be employed to remove from between about 10-15 percent of the color in the waste water effluent from a kraft pulp mill and its bleach plant. Thus, between 85 and 90 percent of the colored material from a kraft pulp mill and its bleach plant will pass through the bio-chemical degradation step into surface water disposal.
The purification of pulp mill effluents pose a unique problem in waste water treatment because the effluents are less amenable to conventional water treatment procedures known and applied today in municipal waste water purification. Because pulp mill effluents contain trace metals and chemical compounds that resist biological degradation, the first line of attack in water pollution abatement resides in the chemical treatment applied to the pulp to achieve the desired product. Any change in the actual processing of pulp treatment which provides an effluent containing fewer contaminants carries with it the reduced requirements for effluent treatment before discharge of wastes to surface water. This first line of attack, the actual chemical bleach technique employed, poses the combined problems of economics, effectiveness of the chemical treatment, as well as the achievement of the desired bleached product characteristics demanded by the pulp and paper industry, and is critical in any active program of water pollution abatement. Thus, although changes in the chemical processing of pulp may not be considered a panacea for water pollution abatement, dramatic improvements in pulp mill bleaching are essential to achieve those desired goals.
One conventional bleach sequence employed as a multi-stage bleaching process for soft wood pulp involves the initial treatment of the cellulosic pulp with chlorine followed by a second stage caustic extraction, a third stage hypochlorite bleach and a fourth stage chlorine dioxide bleach stage. This multi-stage bleach process is conventionally designated CEHD, the letters respectively representing chlorination, extraction, hypochlorite treatment, and a chlorine dioxide treatment. The extraction stage of this bleach sequence is responsible for about 88 percent of the total color coming from the entire multi-stage bleach process. In a laboratory scale application of the bleach sequence CEHD to soft wood pulp it was discovered that the amount of colored bodies appearing in the bleach liquors after each stage of treatment, corrected as to volume, was approximately 332 ppm after chlorination, 2953 ppm after extraction, 23 ppm after hypochlorite treatment and 39 ppm after chlorine dioxide application, for total color body content of the effluent from the bleach sequence of 3,347 parts per million.
Although the effluent from a soft wood bleaching process is much more highly colored than the effluent from hardwood bleaching by the same process, either effluent is objectionable. It was discovered, through application of the bleach sequence CEHD to a hardwood pulp that colored body effluent contamination after each stage of treatment, corrected as to volume, was approximately 227 after chlorination, 809 after extraction, 27 after hypochlorite treatment and 78 after chlorine dioxide application, for a total color body content of the effluent from the bleach sequence of 1,141 parts per million.
Regional and state efforts to control the color of discharge pulp mill effluents have placed a standard of water quality upon discharged effluents at about 50 parts per million APHA (American Public Health Association) as a near term goal. Hence, any contribution which will assist in reaching this goal represents a decided advance in the art of pollution control.